1. Field of the Invention
The present invention relates to the cleaning of a semiconductor wafer and more specifically the cleaning of a semiconductor wafer exhibiting, after plasma etching, traces of a brominated residual polymer.
2. Discussion of the Related Art
The manufacturing of integrated circuits from silicon wafers requires many steps of etching and implantation of various materials according to some predetermined patterns. A specific step consists of creating, on a thin silicon oxide layer, patterns formed of polysilicon. For this purpose, a uniform polysilicon wafer is deposited over the entire surface of the silicon oxide. This polysilicon layer is covered with a uniform resist layer, in which are formed the patterns which are desired to be reproduced on the polysilicon, by a conventional photolithographic etching method.
The wafer is then submitted to a plasma etching which will etch the polysilicon anisotropically in a direction substantially perpendicular to the wafer surface and form the desired patterns in the polysilicon. This plasma etching operation consists of digging vertical holes of various cross-sections in the polysilicon. At the end of the etching, it can be seen that a thin film especially formed of a polymer including bromine, chlorine, silicon, and carbon, has been deposited on the substantially vertical walls of the hole.
The following step of the manufacturing process consists of an ion implantation during which the existence of a resin layer above the polysilicon patterns is required. The presence of the substantially conductive brominated film on the hole walls as well as on the oxide surface is then undesirable and substantially decreases the manufacturing output, that is, the number of good circuits obtained with one wafer.
A conventional method to remove the polymer film consists of completely cleaning the surface of the silicon wafer, that is, completely removing the resin, then performing a hydrofluoric acid etch to remove the polymer traces, then reinstalling a resin layer at the desired locations. This solution requires a great number of steps and one of them, that is, the reinstallation of the resin at the desired locations after cleaning, is particularly difficult for reasons of at least mask repositioning.
Another conventional method consists of rotating the wafer in its plane around its axis, at a rotation speed of approximately 100 CPM; submitting it for a predetermined duration to an atmosphere composed of hydrofluoric acid, water vapor, and nitrogen; then rinsing by means of a water squirt. A major disadvantage of this method is that the proportions between hydrofluoric acid in gas form and water vapor, as well as the time of exposure of the silicon wafer to this atmosphere are parameters which are difficult to adequately adjust for each processed batch of wafers. This method often results either in an incomplete removal of the polymer film, or in an at least partial delamination of the resin layer. There exists between the resin and the polysilicon a thin layer of a adherence-enhancing material which seems to be rapidly deteriorated by a cleaning according to this method. Further, this method results in an erosion of the silicon oxide layer coming close to 30 nanometers. In the context of technologies using a relatively thin oxide, such an oxide erosion can be excessive and result in undesirable modifications of the electric behavior of the elements of the circuit using the silicon oxide layer. Further, when the silicon oxide layer is very thin, there is a high risk of piercing thereof.